This invention relates to switching circuits and in particular to nonlinear resistor control circuits and their applicability to devices for display or printing.
Of the various forms of flat-panel displays being developed for television and other electronic applications, those making use of liquid crystal technology constitute one of the most important types. Much of the interest in such displays derives from their low voltage, low current requirements as well as their low cost and relatively long operating life. In their simplest form, such displays consist of a matrix of twisted nematic liquid crystal elements addressed by means of voltages applied to them by a set of X and Y electrodes. As is well known in the art, however, while such displays can produce satisfactory images when the number of rows of picture elements is limited, their contrast and viewing angle becomes progressively smaller as the number of rows of picture elements is increased. In practice, satisfactory display panels of this type can be made which contain less than about 100 to 200 rows of elements. Although the number of rows can be somewhat increased by using some of the newer forms of liquid crystals materials, such as, the "super-twisted" and ferroelectric types, the long response time of the former and the absence of gray scale of the latter make these alternative approaches unsuitable for television and other applications where images involving motion and containing gray scale are required.
To overcome the limitations of X-Y addressed twisted-nematic displays and allow their use in high-resolution or high-definition-television (HDTV) displays containing a large number of rows of picture elements, a variety of "active matrix" schemes have been developed in which one or more, semiconductor elements are provided for each liquid crystal element. In all these schemes, the semiconductor elements serve to more efficiently block the addressing signals from reaching unselected liquid crystal elements during the line-by-line addressing cycle. In addition, they prevent the capacitive electric charges established across the liquid crystal elements by the signal voltages during addressing from rapidly leaking off the elements between successive addressing cycles, thus increasing the amount of light modulation produced.
The semiconductor elements used for this purpose are generally in the form of field-effect transistors or diodes. These are fabricated from a material such as silicon in amorphous or polycrystalline form to enable the deposition of multi-element arrays at relatively low temperature on a large area substrate. Although displays making use of field effect transistors have received the most attention until now, various factors make this approach difficult. Among these are obtaining a high yield of transistors with the desired electrical characteristics, instability and drift of the transistor characteristics with time, and short circuits occurring at the many crossover points of the conducting lines required for such large arrays of transistor and liquid crystal elements. Since many of these problems are reduced or eliminated when two terminal semiconductor elements are employed, there has been a growing interest in the use of such semiconductor elements in place of the three terminal transistor elements.
Of the various active matrix display devices which incorporate two terminal semiconductor elements, several make use of one or more rectifying diode elements at each liquid crystal element. In one arrangement, as described, for example, in the paper "A Novel Back-to-Back Diode Element for Addressing LCDs" by T. Sato et al., in the 1987 Digest of the Society for Information Display, pp. 59-61, a pair of back-to-back rectifying diodes is connected in series with each liquid crystal element. In another arrangement, a pair of parallel diodes of opposite polarity is connected in series with each liquid crystal element as described in the paper, "An LC-TV Display Controlled by a-Si Diode Rings" by S. Togashi et al., in the Proceedings of the SID, Vol. 26(1), pp. 9-15, 1985. In a third arrangement, two diodes of the same polarity are connected in series with each other, with the junction of these diodes connected to the liquid crystal element, as described in the paper, "A New Amorphous-Silcon Alloy PIN Liquid Crystal Display" by Z. Yaniv et al. in the 1986 Digest of the Society for Information Display, pp. 278-280. In place of rectifying elements, other schemes have also been explored in which an electrically-symmetrical nonlinear resistive element is connected in series with each liquid crystal element. An example of such a scheme is described in the paper, "The Optimization of Metal-Insulator-Metal Nonlinear devices for Use in Multiplexed Liquid Crystal Displays" by D. R. Baraff et al. in the IEEE Transactions on Electron Devices, Vol. ED-28, pp. 736-739, 1981.
In all of the above schemes, the fabrication of a large area display, for example, 12".times.12" is size or larger, poses several special problems. Aside from the increasing difficulty of avoiding defective circuit elements as the number of picture elements is increased, the costs of panel fabrication are greatly increased since fabrication of the required diode semiconductor elements requires the use of vacuum systems specially designed for processing such large substrates. In addition, since the glass plates confining the liquid crystal layer must be maintained at a uniform spacing, for example, 5 .mu.m over the entire area, further fabrication problems are encountered.
It is an object of this invention to simplify the basic structure of a display panel or printing array to lower the cost of fabrication and reduce the possibility of defects in a multi-element large area device. Another object of this invention is to enable the fabrication of the semiconductor circuit elements of an image generating device in a normal room environment, thus avoiding the need for processing the device in a vacuum system. Still another object of this invention is to enable the fabrication of a display device on a thin layer of plastic material in which tiny liquid crystal cells are encapsulated, thus avoiding the difficulties of providing conventional liquid crystal layers with uniform thickness over a large area.